Vortex ring state
The vortex ring state (VRS) is a dangerous aerodynamic condition that may arise in helicopter flight, when a vortex ring system engulfs the rotor causing severe loss of lift. Vortex ring state is sometimes referred to as settling with power, though the latter is also a terminology variously used for low-RPM states caused by rapid increases in collective resulting in over-pitching of the main rotor blades and/or from exceeding engine capability during rapid deceleration and/or descent. The Federal Aviation Administration (FAA) sees these terms as synonymous, whereas Transport Canada sees them as two different phenomena.
This condition also occurs with tiltrotors, and was responsible for an accident involving a V-22 Osprey. Vortex ring state caused the loss of a heavily modified MH-60 helicopter during Operation Neptune Spear, the 2011 raid in which Osama bin Laden was killed.
In forward flight, there is no upward flow (upflow) of air in the hub area. As forward airspeed decreases and vertical descent rates increase, an upflow begins because there are no airfoil surfaces in the mast and blade grip area. As volume of upflow increases, the induced flow (air pulled or "induced" down through the rotor system) of the inner blade sections is overcome and the blades begin to stall near the hub. As the inner blade sections stall, a second set of vortices, similar to the rotor tip vortices, form in the center of the rotor system, and combined with the outer set of vortices, result in severe loss of lift. The failure of a helicopter pilot to recognize and react to the condition can lead to high descent rates and ground impact.
Occurrence
A helicopter normally encounters this condition when attempting to hover out of ground effect above the hovering ceiling for the aircraft, hovering out of ground effect without maintaining precise altitude control, and while making downwind or steep, powered approaches when the airspeed drops to nearly zero.
Detection and correction
The signs of VRS are a vibration in the main rotor system followed by an increasing sink rate and possibly a decrease of cyclic authority.
In single rotor helicopters, the vortex ring state is traditionally corrected by slightly lowering the collective to regain cyclic authority and using the cyclic control to apply lateral motion, often pitching the nose down to establish forward flight. In tandem-rotor helicopters, recovery is accomplished through lateral cyclic or pedal input or both. The aircraft will fly out of the vortex ring into "clean air", and will be able to regain lift.
Another correction now widely known as the Vuichard Recovery Technique after gaining recent popularity, was taught by Claude Vuichard, a Federal Office for Civil Aviation (FOCA) inspector in Switzerland. This technique uses a combination of all three controls together to reduce altitude loss and recover more quickly: apply cyclic in the direction of tail rotor thrust, increase the collective to climb power, and coordinate with the power pedal to maintain heading (cross controls). Recovery is complete when the rotor disc reaches the upwind part of the vortex.
Powering out of vortex ring state
It is possible to power out of vortex ring state, but this requires having about twice the power it takes to hover. Only one full-scale helicopter, the Sikorsky S-64 Skycrane, is documented as being able to do this, when unladen.